Acetolactate synthase (hereinafter referred to as “ALS”) is a rate-limiting enzyme in the biosynthetic pathway of branched chain amino acids, such as leucine, valine and isoleucine, and is known as an essential enzyme for the growth of plants. ALS is also known to be present in a wide variety of higher plants. In addition, ALS is found in various microorganisms, such as yeast (Saccharomyces cerevisiae), Escherichia coli, and Salmonella typhimurium. 
Three types of isoenzymes of ALS are known to be present in Escherichia coli and Salmonella typhimurium. Each of these isoenzymes is a hetero oligomer consisting of catalytic subunits with a large molecular weight that govern catalytic activity of the enzyme and regulatory subunits with a small molecular weight that function as feedback inhibitors by binding of branched-chain amino acids (Chipman et al., Biochim. Biophys. Acta. 1385, 401-419, 1998). Catalytic subunits are located at Ilv IH, Ilv GM and Ilv BN operons, respectively. On the other hand, ALS in yeast is a single enzyme, which comprises a catalytic subunit and a regulatory subunit, as is the case in bacteria (Pang et al., Biochemistry, 38, 5222-5231, 1999). The catalytic protein subunit is located at the locus ILV2.
In plants, ALS is known to consist catalytic subunit(s) and regulatory subunit(s) as is the case in the above microorganisms (Hershey et al., Plant Molecular Biology. 40, 795-806, 1999). For example, the catalytic subunit of ALS in tobacco (dicotyledon) is coded by two gene loci, SuRA and SuRB (Lee et al., EMBO J. 7, 1241-1248, 1988); and that in maize is coded by two gene loci, als 1 and als 2 (Burr et al., Trends in Genetics 7, 55-61, 1991; Lawrence et al., Plant Mol. Biol. 18, 1185-1187, 1992). The nucleotide sequences of genes coding for a catalytic subunit have been completely determined for dicotyledonous plants including tobacco, Arabidopsis, rapeseed, cotton, Xanthium, Amaranthus and Kochia (see Chipman et al., Biochim. Biophys. Acta. 1385, 401-419, 1998 and domestic re-publication of PCT international publication for patent applications WO97/08327). However, maize and rice (Kaku et al., the 26th Conference of Pesticide Science Society of Japan, Lecture Abstracts, p 101, 2001) are the only monocotyledonous plants whose nucleotide sequences have been completely determined.
Meanwhile, herbicides, for example, sulfonylurea herbicides, imidazolinon herbicides, triazolopyrimidine herbicides and pyrimidinyl carboxy herbicides (hereinafter referred to as “PC herbicides”), are known to suppress the growth of a plant by inhibiting ALS (Ray, Plant Physiol. 75, 827-831, 1984; Shaner et al., Plant Physiol. 76, 545-546, 1984; Subramanian et al., Plant Physiol. 96, 310-313, 1991; Shimizu et al., J. Pestic. Sci. 19, 59-67, 1994).
As shown in Tables 1 and 2, known plants having resistance to these herbicides contain a gene coding for ALS that includes substitution of one or two nucleotides which induces substitution of one or two amino acids in a region conserved among different species.
TABLE 1Mutation in plant ALS which imparts resistance against ALS-inhibiting type herbicide (1)HerbicideCorresponding rice Plant speciesMutationtested ALS amino acidZea maysAla90ThrIMAla96ThrArabidopsis thalianaAla122ValAla96ValXantium strumariumAla100ThrIMAla96ThrBeta vulgarisAla113ThrIM/SUAla96ThrArabidopsis thalianaMet124GluMet98GluArabidopsis thalianaMet124IleMet98IleArabidopsis thalianaMet124HisMet98HisLactuca serriolaPro→HisSUPro171HisKochia scopariaPro189ThrSUPro171ThrKochia scopariaPro189SerSUPro171SerKochia scopariaPro189ArgSUPro171ArgKochia scopariaPro189LeuSUPro171LeuKochia scopariaPro189GlnSUPro171GlnKochia scopariaPro189AlaSUPro171AlaBrassica napusPro173SerPro171SerNicotina tabacumPro196GlnSUPro171GlnNicotina tabacumPro196AlaSUPro171AlaNicotina tabacumPro196SerSUPro171SerArabidopsis thalianaPro197SerSUPro171SerArabidopsis thalianaPro197deletionPro171deletionBeta vulgarisPro188SerIM/SUPro171SerSisymbrium orientalePro→IlePro171IleBrassica tournefortiiPro→AlaPro171AlaScirpus juncoidesPro→LeuSUPro171LeuScirpus juncoidesPro179AlaSUPro171AlaScirpus juncoidesPro179GlnSUPro171GlnScirpus juncoidesPro179SerSUPro171SerScirpus juncoidesPro179LysSUPro171LysLindernia micranthaPro→GlnSUPro171GlnLindernia procumbensPro→SerSUPro171SerLindernia dubia subsp.Pro→SerSUPro171SerLindernia dubiaPro→AlaSUPro171AlaArabidopsis thalianaArg199AlaArg173AlaArabidopsis thalianaArg199GluArg173GluXantium strumariumAla183ValAla179ValArabidopsis thalianaPhe206ArgPhe180Arg
TABLE 2Mutation in plant ALS which imparts resistance to ALS-inhibiting type herbicide (2)HerbicideCorresponding rice Plant speciesMutationtestedALS amino acidKochia scopariaAsp260GlySUAsp242GlyKochia scoporiaTrp487ArgSUTry465ArgKochia scopariaAsn561SerSUAsn539SerKochia scopariaTrp570LeuTrp548LeuGossypium hirsutum L.Trp563CysSU ?Try548CysGossypium hirsutum L.Trp563SerSU ?Try548SerBrassica napusTrp557LeuTry548LeuZea mays L.Trp552LeuIMTry548LeuNicotina tabacum L.Trp537LeuSUTry548LeuArabidopsis thalianaTrp574LeuTry548LeuArabidopsis thalianaTrp574SerTry548SerArabidopsis thalianaTrp574deletionTry548deletionXantium strumariumTrp552LeuIMTry548LeuOryza sativa.Trp548LeuPCTry548LeuAmaranthus sp.Trp569LeuTry548LeuAmaransus rudisTrp569LeuIMITry548LeuSisymbrium orientaleTrp→LeuTry548LeuZea maysSer621AspIMSer627AspZea maysSer621AsnIMSer627AsnArabidopsis thalianaSer653AsnIMSer627AsnArabidopsis thalianaSer653ThrSer627ThrArabidopsis thalianaSer653PheSer627PheArabidopsis thalianaSer653delitionSer627deletionOryza sativaSer627IlePCSer627IleKochia scopariaVal276GluSU
Examples of such a gene include a gene coding for ALS having resistance specific to sulfonylurea herbicides (see Kathleen et al., EMBO J. 7, 1241-1248, 1988; Mourad et al., Planta, 188, 491-497, 1992; Guttieri et al., Weed Sci. 43, 175-178, 1995; Bernasconi et al., J. Biol. Chem. 270, 17381-17385, 1995; and JP Patent Publication (Unexamined Application) No. 63-71184); a gene coding for ALS having resistance specific to imidazolinon herbicides (see Mourad et al., Planta, 188, 491-497, 1992; Lee et al., FEBS Lett. 452, 341-345, 1999; and JP Patent Publication (Unexamined Application) No. 5-227964); a gene coding for ALS having resistance to both sulfonylurea and imidazolinon herbicides (see Kathleen et al., EMBO J. 7, 1241-1248, 1988; Bernasconi et al., J. Biol. Chem. 270, 17381-17385, 1995; Hattori et al., Mol. Gen. Genet. 246, 419-425, 1995; Alison et al., Plant Physiol. 111, 1353, 1996; Rajasekarau et al., Plant Sci. 119, 115-124, 1996; JP Patent Publication (Unexamined Application) No. 63-71184; JP Patent Publication (Unexamined Application) No. 4-311392; and Bernasconi et al., U.S. Pat. No. 5,633,437, 1997); and a gene coding for ALS having a high level of resistance to PC herbicides (Kaku et al., the 26th Conference of Pesticide Science Society of Japan, Lecture Abstracts, p. 101, 2001). The production of a plant body showing resistance to both sulfonylurea and imidazolinon herbicides has been attempted by crossing a plant having ALS showing resistance specific to sulfonylurea herbicides with a plant having ALS showing resistance specific to imidazolinon herbicides (Mourad et al., Mol. Gen. Genet, 243, 178-184, 1994). Furthermore, artificial alteration of a gene coding for ALS into a herbicide resistance gene has been attempted (see Ott et al., J. Mol. Biol. 263, 359-368, 1996, JP Patent Publication (Unexamined Application) No. 63-71184, JP Patent Publication (Unexamined Application) No. 5-227964, JP Patent Publication (PCT Translation) No. 11-504213), such that it has been found that a single amino acid deletion causes ALS to show resistance to both sulfonylurea and imidazolinon herbicides (see JP Patent Publication (Unexamined Application) No. 5-227964).
As described above, ALSs having resistance to herbicides, and genes coding for ALS have been aggressively studied. However, only a few cases have been reported concerning a mutant ALS gene having resistance specific to a PC herbicide using resistance to PC herbicides as an indicator. Moreover, there have been also only a few cases reported concerning the study of the resistance to PC herbicides and other herbicides.